Venturi-type mixing devices, often known as eductors, use a principle discovered by Daniel Bernoulli (1700-1782) and are used for applications involving mixing of two liquids. In general, an eductor uses a stream of first liquid flowing from a (usually) pressurized source to a primary inlet thence through a venturi. A second inlet passage extends between the venturi and a container holding a second liquid to be mixed with the first. Often the first liquid is water and the second liquid is a chemical product.
As but one example of how eductors are used to mix water and chemical products, members of building custodial staff often use dispensing equipment which contains one or more different liquids in concentrated form. Such concentrated liquids are in separate containers in the equipment or connected to such equipment. The equipment includes eductor(s) to mix water and a concentrated liquid to form a dilute solution, e.g., a cleaning liquid.
The difference in pressure between that of the concentrate container and that in the eductor venturi urges the second liquid into the path of the high-velocity primary liquid and the liquids are thereby mixed. The resulting dilute solution is directed to a vessel, e.g., a pail used by custodial staff for cleaning. Merely as examples, concentrated liquids may include a neutral cleaner, a "spray-and-wipe" cleaner/degreaser and a glass cleaner.
A manufacturer of dispensing equipment (sold under the trademark SOLUTIONS CENTER.RTM. and other trademarks) and liquid concentrates used therewith is S.C. Johnson & Son, Inc. of Racine, Wis., the assignee of the invention. An eductor of the type used in SOLUTIONS CENTER.RTM. equipment is described in U.S. Pat. No. 5,544,810 (Horvath, Jr. et al.) which is incorporated herein by reference.
Examples of eductor-type mixing devices are disclosed in U.S. Pat. Nos. 3,072,137 (McDougall); 3,166,086 (Holmes); 4,697,610 (Bricker et al.); 5,159,958 (Sand): 5,253,677 (Sand); 5,529,241 (Horvath, Jr. et al.), in PCT International Application Publication No. WO95/34778 (Nowicki et al.) and in other patent documents. The proportioner of the Bricker et al. patent divides the incoming liquid stream into two flow paths, i.e., a primary path through the venturi and a secondary path through two parallel passages. Such passages diverge in a downward direction and liquid flowing therethrough is combined in a cylindrical region with the solution flowing out of the venturi.
The Nowicki et al. PCT application involves a proportioner similar to that of the Bricker et al. patent. Such proportioner has a venturi system, the upper venturi nozzle of which includes three tapered flats rather than the opposed flat sides used in the Bricker et al. proportioner.
The eductor of the Sand '958 patent has passages parallel to the venturi. Water which splashes away from the eductor nozzle and is deflected by the splash plate runs down such passages and past the venturi to be joined with the solution flowing from such venturi. The parallel passages radially outward from the venturi in the eductor of the Sand '677 patent perform a similar "splash-draining" function.
While the devices of these and other prior art patents have been generally satisfactory for their intended purposes, they are not without some disadvantages. One disadvantage involves the matter of mixture foaming. If the dilute solution is excessively foamed, the vessel receiving such solution may overflow with foam and yet contain only a modest quantity of liquid solution.
While not wishing to subscribe to any particular theory as to why certain prior art devices cause excessive foaming, it is believed that aeration of the primary liquid stream may be a significant factor. Another factor may involve joining liquids flowing along two flow paths at high velocity.
Considering the Bricker et al. patent, it is noted that the volume of water flowing down the diverging parallel passages forming the secondary path and/or the above-mentioned cylindrical region may be insufficient to "seal" against the passage walls and prevent air entry. Aeration may result.
Considering the eductor of the Sand '958 patent, the quantity of liquid flowing through the splash-draining passages is unlikely to fill the entirety of the open area below such passages. This may also encourage aeration. And the eductor of the Sand '958 patent flows the primary liquid stream through a disc plate having an enlarged orifice. The resulting space between such stream and the orifice may promote aeration.
The eductor of the Sand '958 patent seemingly has yet other disadvantages. The diameter of the orifice in the disc base is very significantly greater (about 3.5 to 4 times greater) than the diameter of the outlet orifice in the conical portion. To put it another way, the area of the orifice in the disc base is about 12-14 times greater than the area of the outlet orifice. Such outlet orifice seemingly cannot accept any but a very modest flow rate from such disc base orifice. At other than modest flow rates, this configuration apparently causes a good deal of backwardly-directed splashing and is believed to dictate the need to provide a spray shield to at least help prevent spray from exiting the air gap slots.
Yet another disadvantage of certain prior art eductors is that they have inadequate "back pressure tolerance." This is another way of saying that such eductors exhibit undesirably-high pressure drop along their length. (Such pressure drop is sometimes referred to as "insertion loss.")
Such pressure drop can be of concern for the following reasons. Assuming the primary liquid enters the eductor at some maximum pressure, excessive eductor pressure drop results in less pressure available for liquid mixing and, notably, for urging the mixed solution from the eductor outlet. The latter consideration is always important and becomes more so if, for example, a hose connected to the outlet of an eductor is elevated above the eductor or is even pointed upward while mixed liquid is flowing therefrom. Such hose positioning increases back pressure at the eductor outlet. And using an improperly-sized hose and/or a hose of inordinate length also increases eductor outlet back pressure, leaving less pressure available for solution dispensing.
It is noted that the conical opening and converging nozzle mentioned in the Sand '958 and '677 patents, respectively, present relatively long flow passages to a stream of water passing through such passages. And long flow passages impose higher pressure drops, leaving less pressure available for the mixing and dispensing functions.
Another shortcoming of certain prior art eductors is that they are capable of mixing only two liquids. There are instances involving, e.g., dispensing equipment where it would be highly desirable to mix more than two liquids and/or to perform other functions not possible with two inlet eductors.
Yet another shortcoming of certain prior art eductors is that it is difficult to change a performance characteristic, e.g., the level of vacuum "pulled" by the eductor.
Still another characteristic of certain eductors is that they must be oriented vertically. But sometimes vertical orientation is not practical or even possible.
And yet another characteristic of certain prior art eductors is that they are somewhat noisy and operate with a very-audible and characteristic "hissing" sound.
A new mixing eductor which overcomes some of the problems and shortcomings of known eductors would be an important advance in the art.